Transcript
Page 1: Solid-state 51V NMR for characterization of vanadium-containing systems

Catalysis Today 78 (2003) 91–104

Solid-state51V NMR for characterization ofvanadium-containing systems

O.B. Lapinaa,∗, A.A. Shubina, D.F. Khabibulina, V.V. Terskikha,P.R. Bodartb, J.-P. Amoureuxb

a Boreskov Institute of Catalysis, Prosp. Lavrentieva 5, 630090 Novosibirsk, Russiab Laboratoire de Dynamique et Structure des Matériaux Moléculaire, Universite des Sciences et Technologies de Lille,

F-59655 Villeneuve d’Ascq, France

Abstract

This overview paper includes both published and original data of the current state of the field of51V NMR in solid-statechemistry. Advantages and shortcomings of different NMR techniques in their applications to vanadium are discussed onthe examples of their application to various vanadia based systems (including individual highly crystalline compounds, solidsolutions, glasses, catalysts). New correlations between local structure of vanadium atoms and NMR parameters allowingto discriminate at least seven different types of vanadium sites (tetrahedral sites of Q0, Q1 and Q2 types; trigonal pyramidsof 3 = 1 and 3= 2 (V2O5 like) types; tetragonal pyramids of 4= 1, 4 = 2 types) are proposed. It is demonstrated thatcompetent combination of different NMR approaches permits now not only to describe different vanadium sites in highlycrystalline and amorphous materials, but also to insight into the structural aspects of disorder in crystallinity as well as toreveal the behavior of different functional groups at elevated temperatures. The influence of low valence vanadium atoms on51V NMR spectra is also discussed.© 2002 Published by Elsevier Science B.V.

Keywords: Vanadium; Solid-state nuclear magnetic resonance;51V NMR; Ultra-high-speed MAS; MQMAS; SATRAS; STMAS; Individualcompounds; Solid solutions; Glasses; Catalysts

1. Introduction

Among group V elements, vanadium is one of themost important element, widely used in solid-statechemistry, materials science, catalysis and engineer-ing. Nowadays 51V solid-state nuclear magneticresonance (NMR) spectroscopy became a keystonetechnique for characterization of local structure ofvanadium sites in different vanadium systems[1,2].Modern NMR techniques such as ultra-high-speedMAS (35 kHz and higher), MQMAS, SATRAS al-

∗ Corresponding author.E-mail address: [email protected] (O.B. Lapina).

low to obtain direct and precise information on thelocal structure of vanadium sites: (i) the number ofnonequivalent vanadium sites, (ii) coordination num-bers, (iii) the nature of the atoms in the first coordi-nation sphere, (iv) the distortion of this coordinationsphere, (v) association of vanadium–oxygen poly-hedron. In addition, spin echo mapping spectra orultra-high-speed MAS experiments can highlight V5+atoms bounded via oxygen atom by V3+ or V4+; de-fects and distortions of the structure can be revealedby analysis of distributions of chemical shielding andquadrupolar tensor parameters. The main purpose ofthis work is to demonstrate current possibilities of51V NMR in solid-state chemistry, that is why there is

0920-5861/02/$ – see front matter © 2002 Published by Elsevier Science B.V.PII: S0920-5861(02)00299-7

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no deep historical excursus in this field, as well as nocomplete bibliography. At present the bibliography insolid-state51V NMR applications counted hundredspapers, the review of them is for the future. In thispaper, we considering only some results obtained bySATRAS, high-speed MAS (35 kHz), MQMAS, andSTMAS, which demonstrate applicability of thesetechniques to various vanadia systems.

2. Modern solid-state 51V NMR technique.Polycrystalline V5+ oxide compounds

51V nucleus (natural abundance 99.76%) has a spinquantum number of72 and an electric quadrupolemoment of 0.05 b, the relative intensity of51V NMRsignal is 0.38 compared to an equal number of pro-tons. In presence of a magnetic field, each vanadiumnucleus of solid diamagnetic samples experiences, ingeneral, three different types of interaction: (i) dipoleinteraction of its magnetic moment with magneticmoments of other nuclei, (ii) quadrupole interactionof its electric quadrupole moment with the electricfield gradient, (iii) chemical shielding anisotropy(CSA) interaction. These interactions individuallybroaden and even shift (in the case of the quadrupo-lar interaction at second order) the observed lines, tosuch an extent that in static powdered solids, NMRhas for long been accepted as of moderate value,until magic angle spinning (MAS) technique was in-troduced. Indeed, the extensive applicability of NMRto solids relies heavily on MAS. This technique isable to narrow the lines by successful averagingdipolar, anisotropic chemical shielding and first orderquadrupolar effects. However, a residual line broaden-ing issued from the quadrupolar interaction at secondorder remains and it has been until 1995 the mainlimitation of MAS resolution. Nevertheless, in theparticular case of vanadium, the small value of elec-tric quadrupole moment moderates the quadrupolarinteraction and simple MAS technique, has proven tobe a very convenient technique for vanadium char-acterization. The SAtellite TRAnsition Spectroscopy(SATRAS) [3,4] method has rapidly taken advan-tage of the MAS high-resolution spectra availablefor vanadium-containing sample. This technique al-lows a simple determination of the isotropic chemicalshift (δiso) and composite quadrupolar coupling con-

stant (λ = CQ

√1 + η2/3, CQ = e2qQ/h). In

combination with automatic analysis (refinement)of the intensities of well-resolved satellite spinningsidebands the complete set of quadrupolar and CSAtensor parameters as well as their relative orientationscan be measured. Thanks to SATRAS and spectralsimulation, precise NMR data have been obtained formost of the individual vanadium-oxide compounds ofthe system V2O5–MxOy (M = mono-, di-, tri- andtetra-valent metals)[3–10].

Seven types of vanadium sites could be recog-nized based on the values of chemical shielding andquadrupolar tensors parameters obtained by SATRASmethod for the above mentioned highly crystallineindividual compounds with well known structures[3–10].

Tetrahedral vanadium sites of Q0, Q1 and Q2 typescould be revealed using correlation between the type(ησ —chemical shielding asymmetry parameter) andvalue (�σ—CSA) of chemical shielding anisotropy[1,2,11] (Fig. 1):

(i) Vanadium in regular tetrahedral oxygen environ-ment (Q0 type) has almost spherically symmet-ric chemical shielding tensor with small valueof anisotropy (�σ < 100 ppm); quadrupolarconstantCQ varies from 1 to 6 MHz; chemicalshielding asymmetry parameter varies from 0 upto 1 [3,4,10].

(ii) Vanadium in slightly distorted tetrahedral siteswith the adjacent tetrahedra sharing one com-mon oxygen atom (Q1 type) has an asymmetricchemical shielding tensor, but with larger value ofanisotropy (100< �σ < 200 ppm); quadrupo-lar constant varies from 2.5 to 10 MHz; chemicalshielding asymmetry parameter changes from 0.1to 0.9[3,4,10].

(iii) Vanadium in strongly distorted tetrahedral siteswith adjacent tetrahedra sharing two commonoxygen atoms (Q2 type) has an asymmetricchemical shielding tensor with large value ofanisotropy (200< �σ < 500 ppm); quadrupo-lar constant varies from 2 to 7 MHz; chemicalshielding asymmetry parameters changes from0.6 to 0.8[3,4,10].

Whereas it is clear that vanadium sites indifferent pyramid couldn’t be determined fromFig. 1. These sites could be recognized using

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Fig. 1. Correlation between51V asymmetry (ησ ) and anisotropy parameters (�σ ) of chemical shielding tensor obtained for various vanadiacompounds.

the correlation betweeneffective σ⊥ estimatedas (σ⊥ ∼1/2(σ1 + σ2), σ i—components ofCS-tensor) and quadrupolar coupling constant(CQ) for the case of a large value of CSA(200 ppm<�σ ) andησ < 0.6 (Fig. 2):

(iv) Vanadium in tetragonal pyramid of 4= 2 typehas an axially symmetric chemical shielding ten-sor with large value of anisotropy (200< �σ <

Fig. 2. Correlation between51V NMR chemical shielding tensor component (σ⊥) and quadrupolar coupling constant (CQ) for vanadiumpolyhedra with V=O bond having(200 ppm< �σ) andησ < 0.6.

500 ppm); quadrupolar constant varies from 4 to8 MHz; chemical shielding asymmetry parame-ters changes from 0 to 0.6,σ⊥ ∼ 200–400 ppm[5–10].

(v) Vanadium in tetragonal pyramid of 4= 1 typehas an axially symmetric chemical shieldingtensor with a large value of anisotropy (400<�σ < 550 ppm); quadrupolar constant varies

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from 1 to 3 MHz; chemical shielding asymme-try parameters changes from 0 to 0.2,σ⊥ ∼200−400 ppm[5–10].

(vi) Vanadium in trigonal pyramid of 3= 1 type hasan axially symmetric chemical shielding tensorwith large value of anisotropy (200< �σ <

500 ppm); quadrupolar constant varies from 1 to4 MHz; chemical shielding asymmetry parame-ters changes from 0 to 0.6,σ⊥ ∼ 400–600 ppm[5–10,12].

(vii) Vanadium in V2O5 like species (distorted octa-hedral or trigonal bipyramidal of 3= 2 type)has an axially symmetric chemical shielding ten-sor with large value of anisotropy(500 ppm<

�σ); quadrupolar constant varies from 0 to3 MHz; chemical shielding asymmetry parame-ters changes from 0 to 0.1,σ⊥ ∼ 200–350 ppm[5–10].

Thus based on the correlations presented inFigs. 1and 2seven different types of vanadia sites could bedetermined in vanadia systems (Fig. 3).

However, SATRAS has its own limitations, whichare inherent to the resolution of the MAS tech-nique. Spectra composed of nonequivalent vanadiumsites with overlapping spinning sidebands may bedifficult to analyze. In addition, defects and dis-tortions of the structures broaden both, static andMAS spectra and further complicate the analysis.

Fig. 3. The structures of vanadium sites, which could be determined from51V NMR parameters.

In these cases alternative techniques can be highlyrecommended. Spin-averaging approaches in com-bination with MAS have recently opened a new di-mension in the spectroscopy of quadrupolar nuclei.In particular, the two-dimensional multiple quantumMAS (MQMAS) technique[13,14] correlates mul-tiple quantum coherences with the single quantumcoherence of the same spin (the central transition),thus achieving a high-resolution spectrum free ofsecond-order broadening effects in one dimension.From the two-dimensional data matrix, the compos-ite quadrupolar coupling constant and the isotropicchemical shift can easily be extracted. The simplicityof this technique has made it notably popular andapplications have been reported on a variety of nu-clei including sodium, rubidium, aluminum, oxygen,boron, niobium, cobalt, and numerous species withdifferent spin quantum numbers that are characterizedby a variety of coupling environments. However, it iscommonly admitted that small value of quadrupolarmoment and large value of CSA are a limitation toMQMAS and, in particular, prevent its application tovanadium nucleus[15].

Recently, we have demonstrated that a combina-tion of ultra-high rotation frequency (30–35 kHz)with low power radio-frequency excitation allows theapplication of MQMAS to vanadium nucleus at allvanadium coordination[16]. For complex spectra withoverlapping lines, MQMAS has significant privilege

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Fig. 4. 51V 3QMAS spectra of AlVO4 (a), Ba2V2O7 (b), Cs2V4O11 (c), Rb2V6O16 (d). The experiments were recorded at a magnetic fieldof 9.4 T. A 2.5 mm MAS probe was used at spinning frequencies in the range 30–35 kHz. The three-pulse Z-filter sequence was employedfor the recording of pure absorption 3QMAS spectra and 288 transients were recorded with 64 increments in the indirect dimension[16].

over SATRAS.Fig. 4 illustrates51V 3QMAS spec-tra of AlVO4, Ba2V2O7, Cs2V4O11 and Rb2V6O16,the crystalline structures of these compounds revealfrom two to three nonequivalent vanadium sites. Theresulting overlapping spinning sidebands observedin MAS, limit and sometime preclude SATRAS ap-plication for all these compounds, while the high

resolution obtained in the 3QMAS spectrum allow aprecise quantification of the isotropic chemical shiftandλ parameter.

Fig. 4a shows the51V 3QMAS spectrum ofAlVO4. The crystalline structure is composed ofthree nonequivalent vanadium sites of type Q0. High-resolution 3QMAS spectrum allows a precise quantifi-

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Table 1Composite quadrupolar coupling constant (λ) and isotropic chem-ical shift (δiso) measured by 3QMAS and SATRAS techniquesa

λ (MHz) δiso (ppm)

3QMAS 1D 3QMAS 1D

AlVO4 4.9 (0.7) 2.48b −659.2 (1.4) −6623.5 (0.9) 2.48b −741.4 (1.4) −7433.8 (0.8) 2.48b −773.2 (1.4) −776

LaVO4 6.0 (0.5) 5.6 −604.8 (1.4) −609

NH4VO3 3.1 (1.0) 2.95 −564.7 (1.4) −563.7

Rb2V6O16 2.47 (1.3) 3.12 −514.3 (1.4) −515<1.8 (1.8) 2.37 −547.6 (1.4) −547

Cs2V4O11 1.9 (1.7) 1.29 −511.9 (1.4) −5132.6 (1.3) noc −569.8 (1.4) no2.4 (1.4) 1.81 −577.7 (1.4) −575

Ba2V2O7 2.27 (1.4) 2.25b −579.6 (1.4) −5793.5 (0.9) 2.25b −587.8 (1.4) −5891.1 (2.9) 2.25b −599.6 (1.4) −600

a For the 3QMAS experiments, errors are reported in brackets.b Theλ value could not be accurately determined by SATRAS

and was estimated.c Not observed.

cation of the isotropic chemical shift andλ parameterfor each three sites.Fig. 4b shows the51V 3QMASspectrum of Ba2V2O7. In this sample, vanadiumatoms are distributed between three nonequivalentQ1 sites. As in the previous case a fully resolved3QMAS spectrum was obtained (extracted NMR pa-rameters with those obtained from SATRAS are givenin Table 1). Fig. 4c and d shows the 3QMAS spec-tra of two compounds (Cs2V4O11 and Rb2V6O16)containing nonequivalent vanadium atoms with five orsix coordination. In Cs2V4O11 there are three types ofvanadium sites: two in distorted tetrahedral pyramidsand the third one with octahedral coordination. InRb2V6O16 there are two types of vanadium sites, bothin octahedral coordination. The well resolved 3QMASspectra obtained for these compounds allow to deter-mine the isotropic chemical shift and the compositequadrupolar coupling constant. Though in Rb2V6O16both sites are observed, the low composite quadrupo-lar coupling constant of the weak line is not easilymeasured from the spectrum, only a maximum valueof the quadrupolar coupling constant can be estimated.

These examples clearly illustrate the superiority ofMQMAS over SATRAS, when several sites produceoverlapping spinning sidebands. However, when the

quadrupolar interaction is weak (CQ < 1 MHz) theincertitude on its quantification may be large.

Recently, the satellite transition MAS (STMAS)[17,18] technique has been proposed as an alterna-tive method to MQMAS. STMAS is expected to givebetter sensitivity and be less dependant on CSA andstrength of the quadrupolar interaction (sites withlow or high quadrupolar coupling constants should bemore easily observable in STMAS than in MQMAS).The STMAS has significant similitude with bothMQMAS and SATRAS experiments: it correlates in atwo-dimensional spectrum the central transition withthe satellite transitions (m, m−1), and in fact STMASexperiment can be described as a two-dimensionalSATRAS experiment. STMAS experiment requiresa very accurate setting of the magic angle (as forSATRAS). Quantitatively, it has been shown, on a23Na STMAS experiment on Na2SO4, that an angleoffset of 0.016◦ has significant line broadening ef-fects [18]. On conventional probes, such an accurateangle setting is not without inherent technical diffi-culties.Eq. (1)gives the full linewidth�νFl along theisotropic dimension (after shearing) of the STMASspectrum[18]:

�νFl ∼= 3√

2CQ

I (2I + 1)�θ (1)

CQ is the quadrupolar coupling constant(CQ =e2qQ/h) and �θ (rad) the angle offset from themagic angle.Eq. (1)shows that the linewidth�νFl isdirectly proportional to the product of the quadrupolarcoupling constant by the angle offset. This clearly in-dicates that, the stronger is quadrupolar coupling, themore precisely magic angle has to be set. In addition,the spin quantum number being involved at the denom-inator of Eq. (1), the adjustment of the magic angleshould be less demanding as this number increases.These two points are indeed, favorable to vanadiumnuclei. The high spin quantum number of7

2 andthe generally weak quadrupolar coupling constantsobserved for vanadium atoms should permit small de-partures from the exact magic angle position. In prin-ciple, SATRAS spectra are composed of as many linesas the number of (m, m − 1) transitions, and it is, forthe moment, not possible to select one particular satel-lite transition other than by adjusting radio-frequencyfields power in order to emphasize the desired tran-sition. The autocorrelation signal of the central

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transition can be removed by selective presaturation orby subtraction of a spectrum of the solely autocorrela-tion signal[19] but these may be of poor efficiency forremoving unwanted satellite transitions. In the case ofspin 7

2 the STMAS spectrum is composed in principle,of the correlation of central transition (−1

2, 12) with

itself plus three satellite transitions (12–3

2, 32–5

2, 52–7

2).This multiplicity of signal may complicate the inter-pretation of spectra when several sites are involved.

Up to now STMAS has been applied to27Al and23Na, and17O [20] in 17O enriched Mg2SiO4. Hereinwe report the first51V STMAS experiment.Fig. 5shows the51V STMAS spectrum of LaVO4, threesignals are observable: (i) the central transition signal(CT), (ii) the (12–3

2) satellite signal (ST1), (iii) the(3

2–52) satellite signal (ST2). The composite quadrupo-

lar coupling constant and the isotropic chemical shiftextracted from the position of the center of gravityof the ST1 line (λ = 6.2 MHz, δiso = −605 ppm)and those resulting of the frequencies of the center

Fig. 5. 51V STMAS spectrum of LaVO4 obtained after shearing(the shearing has been chosen to produce an isotropic line for theST1 signal in the indirect dimension), showing the different cor-relations: (CT) is the autocorrelation of the central transition, ST1correlates the central transition with the (1

2–32) satellite transition

and ST2 correlates the central transition with the (32–5

2) satellitetransition. The experiment has been recorded in a magnetic fieldof 9.4 T, at a spinning frequency of 10 kHz, 256 transients havebeen accumulated. On both axes positive and negative skylineprojections are shown.

of gravity of ST2 (λ = 6.0 MHz, δiso = −600 ppm)agree with SATRAS and MQMAS data[21]. Theintensity of the correlation line with the (5

2–72) tran-

sition (ST3 signal) in not visible inFig. 5 because itsintensity is too low (about the noise level) and belowthe floor used to draw the contour plots.

3. Vanadium in lower oxidation states

Whilst V5+-containing phases can be easily char-acterized by conventional51V MAS NMR, this is notthe case for materials with vanadium atoms in loweroxidation states. However, information concerning thenature, location and oxidation state of vanadium cen-ters can indirectly be obtained from NMR spectra ofneighboring atoms. Thus, on vanadium–phosphorussystem, Gerstein and coworkers[22] and later Tuelet al. [23] has applied a31P spin echo mapping tech-nique, which allows to evidence vanadium paramag-netic centers by observing a large31P spectral re-gion. For V5+ containing compounds31P chemicalshift is in the range 20–40 ppm; for compounds con-taining V4+ phosphorous chemical shift varies from1600 to 2600 ppm; for V3+ containing compounds31Pchemical shift is near 4650 ppm. These shifts are ofthe Fermi-contact origin, and can be used as a mea-sure of the unpared electron spin density transferredfrom the paramagnetic centers to the31P nucleus.Thus,31P line shift gives direct indication on the num-ber of paramagnetic species in the first coordinationsphere of phosphorus as well as on their oxidationstate.

We have decided to extend the31P spin echo map-ping technique to51V5+, in ultra-high-speed MAS(and large spectral width) conditions, in order to de-tect by observing51V NMR signal of V5+ atoms, theproximity of vanadium atoms in a lower oxidationstate. Bronzes and oxovanadium sulfate have beenused as an example of mixed V5+ and V4+ valencecompounds. NaV6O15 formally contains five V5+ions per one V4+ ion. According to the structural dataeach vanadium ion has two nearest vanadium neigh-bors at 3.06 Å. In this compound51V isotropic chem-ical shift is observed at−900 ppm. For Bi6V3O16(there is one V4+ ion per two V5+ ions) isotropicshift is equal to−1447 ppm[24]. K6(VO)4(SO4)8contains V4+/V5+ pairs of VO6 distorted octahedral

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units, usually possessing one short V=O bond about1.57 Å, and four equatorial bonds around 2.00 Å[25]. Eight different SO42− groups form bidentatebridges between the V4+ and V5+ ions. It is worthnoting that the CSA (635 ppm) and the isotropic shift(−655 ppm) of this compound are close to those ob-served for pure V5+ oxosulfatovanadates (compoundsformed in V2O5–M2S2O7 system). The latter indi-cates that the distance (SO4 bridge) between V4+and V5+ along with the short electron spin relaxationtime for V4+ is sufficient to reduce significantly theinfluence of paramagnetic V4+ on diamagnetic V5+.Thus, 51V NMR line shift gives implicit indicationon the number of V4+ ions in the first coordina-tion sphere as well as on the distance between V5+and V4+.

Different positions concerning the influence ofV3+ on 51V NMR spectra of V5+ ions exist in theliterature. Boyarski et al.[26] claims to have directlyobserved V3+ and V5+ NMR signals in V2O4 (whichis proposed to contain vanadium atoms in both states).On the other hand, Pons et al.[27] states that the pres-ence of V3+ prevents observation of51V resonance.We have checked the51V NMR spectrum of V2O4, at35 kHz a signal significantly shifted to a lower field upto 2100 ppm has been observed. The same spectrumwas observed recently by Jakobsen and coworkers[28] and has been assigned to V4+ coupled to theneighbouring V4+ with the formation of diamagneticsite.

Fig. 6. (A) Relative intensities of51V NMR lines from tetrahedral (1) and octahedral (2) vanadium sites in the solid solutionScNb2(1−x)Ta2xVO9 depending onx; (B) changes ofσ iso for tetrahedral vanadium sites andCQ on 45Sc depending onx.

4. Solid solutions, glasses, melts (structure,dynamics)

Solid-state NMR methodology progresses take ad-vantage of the possibility to probe highly crystallineand/or amorphous phases to give new insights intothe structural aspects of disorder in crystalline andglassy solid solutions. In these systems, ion mobilitycan be characterized over a large timescale by severalNMR approaches. Atomic and molecular mobilityoccurring with correlation times of the order of a mil-lisecond can be monitored from variable-temperaturestatic solid-state NMR. Furthermore, temperatureand frequency dependency measurements of nuclearspin–lattice relaxation rates afford a comprehensivecharacterization of internal dynamics of solids occur-ring on a microsecond timescale.

4.1. Solid solutions MNb2(1−x)Ta2xVO9

Substitutional solid solutions MNb2(1−x)Ta2xVO9with different M andx (for M = La, x = 0, 0.1, 0.16,0.2, 0.25, 0.3; for M= Y, x = 0, 0.1, 0.2, 0.29, 0.38,0.58; for M= Sc,x = 0, 0.3, 0.5, 0.7, 1.0) have beencharacterized by51V SATRAS technique. Variation ofM andx allows structural investigations of solid solu-tions to be made using the replacement of some atomsin the cation sublattice (M) and in the anion sublattice(x). The structure of vanadium anion sublattice is thesame for M= La and Y, while for M= Sc, it becomes

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rather complex. These three elements belong to thesame 3B group and have identical valence (3+), butthey differ by their ionic radii 0.81 Å (Sc3+), 0.97 Å(Y3+), 1.06 Å (La3+). Ionic radius of Sc3+ is 23.6%smaller than the La3+ one, while Y3+ ionic radius isonly 8.5% smaller. This can explain the different anionstructure sublattice formation for Sc solid solutions.For M = La and Y, vanadium sublattice is composedof isolated regular tetrahedra of Q0 type. While forM = Sc, vanadium anion sublattice is represented bythe superposition of distorted octahedral and regulartetrahedral sites. Variation ofx (replacement in anionsublattice) results in monotonic changes in the valuesof chemical shifts and linewidths both for51V and45Sc(Fig. 6). Replacement of Nb by Ta is accompanied bymonotonic changes of relative concentration of octa-hedral and tetrahedral sites in Sc system (Fig. 6) [29].

4.2. M2S2O7–V2O5 and MHSO4–V2O5 (M = Na,K, Cs) glasses, melts: structure and dynamicbehavior of the framework units

Valuable information about glass and melt struc-tures as well as about dynamical behavior offramework units may be obtained from temperature-dependent studies of melt-quenched glasses, whichare accepted to retain the main structural featuresof the melt. Exchange processes observed atTg aresupposed to be the most important for melts. Thisapproach has been chosen for the NMR study ofM2S2O7–V2O5 (M = Na, K, Cs) melts[30–32].

At ambient temperature the static51V NMR spectraof melt-quenched M2S2O7–V2O5 glassy samples aresimilar for a wide range of composition. They showa broad line with an axial anisotropy of the chemicalshielding tensor corresponding to vanadium atoms ina distorted octahedral oxygen environment with oneshort V=O bond. They exhibit a Gaussian-like dis-tribution of the quadrupole coupling constant whichis typical for glassy samples. As the vanadium con-centration increases, chemical shielding tensor pa-rameters distribution also increases indicating a moredisordered local environment of vanadium sites.

23Na,39K and133Cs NMR experiments confirm thelack of long-range order in the glassy samples. Ac-cording to these data cations are randomly distributedover the vanadium sulfate framework. At room tem-perature there is no chemical exchange of cations be-

tween different sites. This exchange is only observedat elevated temperatures, when simultaneously, a re-markable transformation of the anion framework takesplace.

Two different ways of crystallization were revealedduring heating of the melt-quenched glassy samples.The type of crystallization, in the studied glasses,depends on the local and extended arrangement ofstructural units. If the glass structure is similar tothe crystal structure of the compounds formed at agiven composition, the glass–crystalline transition(at Tg) is similar to a solid–solid transition. Then,crystallization requires only moderate changes of theanion framework and cations sublattice. However, ifthe composition of the crystalline compounds dif-fers significantly from the starting glass composition,the glass framework needs essential rearrangement.Then, a glass-to-crystal transition follows throughthe so-called “metastable liquid state”. All these pro-cesses may be easily traced by NMR.

As an example, the51V NMR static spectra,recorded for heated 4Cs2S2O7–V2O5 and 2Cs2S2O7–V2O5 melt-quenched glasses are shown inFig. 7. Atambient temperature both spectra exhibit broadenedlines with axial CSA. When the 4Cs2S2O7–V2O5sample is heated to 200–220◦C the spectrum trans-forms to a narrow symmetric line. This isotropicaveraging of CSA indicates a high mobility ofvanadium-containing species, usually occurring inliquids. A visual observation of the metastable liquidat this temperature (Tg for this sample) supports thisconclusion. Simultaneously, cations mobility sharplyincreases, as follows from the133Cs NMR spectrummeasured at the same temperature. As the sample iskept atTg, the metastable liquid crystallizes into crys-talline Cs2S2O7 and Cs4(VO)2O(SO4)4. Since theCs/V molar ratio of the crystalline compound (Cs/V=2) differs from that of the starting glass composition(Cs/V = 4), crystallization indeed proceeds throughthe metastable liquid state. On the contrary, in the caseof 2Cs2S2O7–V2O5 glass composition, the ratio Cs/V(2) is equivalent to that of Cs4(VO)2O(SO4)4, and theglass-to-crystal transition is fast without detection ofa metastable liquid. As crystallization completes, theanion framework stabilizes and is maintained duringsubsequent heating, up to the melting point. At thesame time in both cases, the cation mobility contin-uously increases with the increasing temperature.

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Fig. 7. 51V NMR spectra of melt-quenched glassy samples: (A) Cs2S2O7–V2O5 (Cs/V = 4); (B) Cs2S2O7–V2O5 (Cs/V = 2) measuredat indicated temperatures. NMR spectra have been measured on a Bruker MSL-400 spectrometer (magnetic field 9.4 T) using homemadehigh temperature NMR probe[30].

Fig. 8 shows the51V spectra of(1 − x)K2S2O7–xV2O5 melts. A narrow line is observed at−587 ppmfor the pure V2O5 melt. The increase of the chemicalshift from −612 ppm (solid) to−587 ppm (liquid),indicates the changes of vanadium coordination fromoctahedral (solid) to tetrahedral (melt). This decreasein the coordination number under melting is typicalfor transition metal oxides.51V NMR relaxationtimes may help to estimate the average size of theoxovanadium species in the V2O5 melt. These speciesare found to exist as short one-dimensional chains ofseveral tetrahedral VO4 units. The chains are smallin size, and quadrupole broadening is most likelyremoved by their fast rotation.

Small amounts of pyrosulfate or hydrogensul-fate added to vanadium pentoxide melts essentiallybroaden the51V NMR line (thus at x = 0.98,linewidth is broadened by about a factor of 4).Conceivably individual vanadia (VO4)n chains arelaced with oxosulfate links coordinated to vanadium.In this case the mobility of (VO4)n chains shouldsharply decrease, resulting in a quadrupole broaden-ing of 51V NMR line. Such broadening in melts withx = 0.1–0.3 completely excludes the observation ofany NMR line, since large bulk three-dimensionallyinteracting species are formed.

Similar behavior is observed in the melts with lowvanadium content. In the pyrosulfate system, vana-

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Fig. 8. 51V NMR spectra of the K2S2O7–V2O5 melts with var-ied vanadium concentrations. NMR spectra have been measuredon a Bruker MSL-400 spectrometer (magnetic field 9.4 T) usinghomebuild high temperature NMR probe[32].

dium may exist (at low concentrations) in the formof monomeric vanadium-oxosulfate complexes (this iscorroborated by some17O NMR measurements). Thefast rotational mobility of such complexes diminishesthe broadening mechanism (quadrupolar broadening)and,51V signals can then be observed. The line posi-tion (chemical shift at about−730 ppm) indicates anoctahedral oxygen coordination of vanadium atoms (itis tetrahedral in pure V2O5 melt). The increase in vana-dium content results in the dimerization and oligomer-ization of the vanadium-oxosulfate units. A large sizeof the oligomeric species prevents observation of their51V NMR spectra due to mobility decrease and effec-tive quadrupolar broadening.

Thus, NMR data show that the type of vanadiumspecies in the melts depends on vanadium concentra-tion. At low vanadium concentration, the formationof monomeric or not associated dimeric complexes islikely in the pyrosulfate system. The increase of the

vanadium concentration above 0.1 leads to associa-tion of the complexes with the formation of dimeric(VO)2O(SO4)4

4− and finally oligomeric species atx > 0.3. Further increase of the vanadium content di-minishes the number of sulfate or pyrosulfate anionscoordinated to vanadium. In the melt of the pure V2O5the chains of VO4 tetrahedra bridged via commonoxygen atoms are retained. As was expected, alkalications are characterized by high diffusion mobilityin the melts.

5. Solid catalysts

Last decade, numerous papers devoted to51VNMR studies of solid vanadia based catalysts havebeen published, demonstrating NMR importance andself-descriptiveness for so complex systems as cata-lysts. It is worth noting that valuable data concerningstructure of different binary catalysts, catalysts modi-fied by different promoters, as well as binary catalystsprepared on mixed oxide supports have been obtained(these results were reviewed partly in the encyclope-dia of nuclear magnetic resonance spectroscopy[2]).Nevertheless, the last few years ‘revolution’ in thesolid-state NMR spectroscopy of quadrupolar nucleiwith half integer spin has revealed new approachesand possibilities for51V NMR, especially in its ap-plication for characterization of different vanadiacatalysts. Herein the possibilities of ultra-high-speedMAS (35 kHz) and MQMAS NMR will be shown onthe example of supported VOx /TiO2 catalysts.

Different methods are used for preparation of modeland real VOx /TiO2 catalysts, among them: depositionof thin V2O5 layers on clean anatase single crystal,grafting of VOCl3 from gas phase or VOCl3 (VOR3)from inert solutions on polycrystalline TiO2, impreg-nation of polycrystalline TiO2 by different solutionscontaining V5+ or V4+ salts, equilibrium impregna-tion at different pH, ultra-high intensity grinding of themixture of initial oxides V2O5 and TiO2, and copre-cipitation of mixed vanadia–titania solutions with spe-cial drying techniques (so-called spray-drying). Thesetechniques result in different bonding between vana-dium and titanium atoms. The gentler interaction isexpecting to occur during the deposition of thin V2O5layers on anatase single crystal. Only surface speciesshould be formed during chemical reaction between

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VOR3 and surface hydroxyl groups (grafting). Whileits is impossible to exclude the formation of bulkvanadium sites during impregnation and milling pro-cedures, bulk vanadium sites should be mainly formedunder coprecipitation techniques. In order to revealthe structure of active sites it is necessary to know thestructure of surface vanadium species and bulk vana-dium sites, as well as the concentration of the surfacevanadium species and their catalytic activity.

Grafting technique is achieved through chemicalreaction between hydroxyl groups of the support sur-face with supported vanadium compound (VOCl3):VOCl3 + n(HO–Ti–) → VOCl3−n(O–Ti–)n, wheren can be 0–3. According to1H NMR data only thebridging OH groups react with chlorine atoms ofVOCl3. Isotropic chemical shift and chemical shield-ing anisotropy measured by51V MAS NMR indicatethat two chlorine atoms are simultaneously involvedin this reaction. As a result vanadium is bounded withTiO2 surface via two oxygen bonds with the forma-tion of tetrahedral surface species, characterized byone short vanadium–oxygen bond and a V–Cl bond.

After hydration–dehydration procedures, threegroups of signal (group I composed of the lines situ-ated between−515 and−530 ppm, group II situatedbetween−560 and−580 ppm and group III between−610 to−660 ppm) are formed (Fig. 9a) [33]. Noteespecially, that for the first time the spectra of sam-ples were measured at spinning frequency of 35 kHz.That allowed definite separation of all the isotropiclines from the spinning sidebands. The high field shiftof the lines of the group III is likely due to the stronginteraction between vanadium and Ti (for example,via two or three bonds). It is possible to suggest thatthe lines of group I correspond to vanadium weaklybound to Ti (for example, via one or two bonds). Eachgroup of signal consists of two or three lines that mayresult from the interaction of supported vanadiumwith OH groups located not only on the densest TiO2plane (0 0 1), but also on cleavage planes other than(0 0 1). Observation of 6–9 nonequivalent vanadiumsites indicates that there should be also an associationof vanadium species.

These lines have distinct behaviors under H2O orNH3 adsorption (confirming the formation of sev-eral different vanadium sites on TiO2 surface). Lowtemperature (20◦C) ammonia adsorption (Fig. 9b)preferentially affects sites of group III. At higher

Fig. 9. 51V MAS NMR spectra of VOx /TiO2 catalyst prepared bygrafting method (νr = 35 kHz), at this rotational frequency thereis no overlapping between isotropic lines and spinning sidebands.only isotropic lines, marked by asterisks are shown: (a) spectrumof initial catalyst; (b) after NH3 adsorption at 20◦C; (c) after NH3

adsorption at 200◦C; (d) after NH3 adsorption at 350◦C; (e) afterrunning in catalytic reaction at 350◦C (a treatment in strongerconditions reverts spectrum (e) to the initial one).

temperatures (Fig. 9c and d) attenuation of the signalsof group I is detected, and low field shifted peak isbecoming less intense. Note that group II presentsthe steadiest response to exposure to NH3. The for-mation of a great number of vanadium species on thecatalyst surface was confirmed also by some DRIFTmeasurements.

The species structures significantly change after cat-alytic reaction: the main line corresponds to vanadiumsites of group I. Integrated intensity of group III peaksdecreases, while that of group II remains unchanged(Fig. 9e). Hydroxylation of group III sites and con-version of group I sites can thus be proposed. Highmobility of the protons on the catalyst surface addi-tionally facilitated by the coarse of reaction supportsthis statement.

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O.B. Lapina et al. / Catalysis Today 78 (2003) 91–104 103

Catalysts prepared by spray-drying method arecharacterized by a strong interaction between initialoxides with the formation of the coherent interfacialboundary between TiO2 and V2O5. Vanadium atomsforming this boundary have an octahedral environ-ment, which is less axial than in V2O5 [34]. Largevalue of quadrupolar constant indicates significantstructural distortions (probably, in the second coor-dination sphere) of the local environment of V5+localized in interfacial boundary in comparison withthe local environment of V5+ ions in bulk V2O5.

The appearance of the common boundary betweenvanadia and titania particles has been noticed inthe samples prepared with ball milling followed bythe thermal treatment, but in this case there is theother stacking geometry. According to51V NMRtwo different types of octahedrally coordinated vana-dium (V5+) species (V5+ (I) and V5+ (II)) stronglybonded to TiO2 are formed in milling samples[35].During milling–calcination processes an increase ofV5+(I) and V5+(II) concentrations is observed withthe appearance of V3+ ions, along with the forma-tion of at least three different types of paramagneticV4+ species. Relative amounts of different V4+ andV5+ species depend on the milling time, presenceof H2O in the system, and subsequent calcinationprocedures (temperature and calcination time). Thus,V5+(I) species are formed predominantly duringmilling, whereas those of V5+(II) after the thermaltreatment. For the structural characterization of thesespecies, complete sets of the quadrupole and chem-ical shielding tensor parameters, including relativetensor orientation, have been estimated by SATRAStechnique. This leads to the conclusion that the octa-hedral environment of vanadium in V5+ (II) speciesis less distorted than in V5+ (I), and in both cases thedistortion is less axial than in V2O5.

Comparison of these data with catalytic activity ofthe samples and with theoretical calculations suggeststhe structure of active sites and also the mechanism ofcatalytic reaction[35].

Bright results were obtained by SATRAS and MQ-MAS techniques for doped vanadia catalysts[36,37].Phosphorous-doped VOx /TiO2 catalysts preparedby the spray-drying method and treated under cat-alytic reaction have been studied by SATRAS, 2Dtriple-quantum, quintuple-quantum MAS NMR, andspin echo mapping methods. The simultaneous deter-

mination of CSA and quadrupole tensor parameters,as well as their distributions, permits to draw a con-clusion on the local environment of vanadium sitesin the catalysts. The formation of a triple V–P–Ticompound in phosphorous-doped VOx /TiO2 catalystshas been revealed. Only one type of slightly distortedtetrahedral vanadium atoms bound via oxygen tophosphorous was found in this compound. The verylarge distribution of the quadrupole coupling constantpoints to the irregular structure of this compound[36].

Sodium-doped binary vanadia–titania catalysts pre-pared by the spray-drying method and treated undercatalytic reaction have been studied using fast MASand two-dimensional triple-quantum MAS NMR[37].Several vanadium-containing species have been iden-tified in a representative set of catalysts of varioussodium contents. Analysis of both23Na and51V NMRspectra has provided relative contents of sodium andvanadium compounds. Similar to potassium-modifiedcatalysts, sodium vanadate is formed at high con-centrations of alkali metal (sodium), while at lowconcentrations of Na a vanadium bronze of NaV6O15composition arises. The preferential formation ofsodium vanadate is the cause of an effective decreasein content of strongly bound vanadium. At the sametime, a part of the sodium readily interacts with resid-ual sulfate ions, and a considerable amount of sodiumsulfate was found with23Na 3QMAS NMR. Theuse of multiple-quantum sodium NMR has allowedthe evaluation of the isotropic chemical shifts andλ

parameters for all of the observed sodium species.

6. Conclusions

The results presented in the paper demonstrateadvantages and limitations of modern solid state51V NMR techniques (SATRAS, high-speed MAS(35 kHz), MQMAS, STMAS). Some examples il-lustrate applications of these techniques to variousvanadium systems, including individual crystallinecompounds, solid solutions, glasses and catalysts.Original correlations of the structure of vanadiumatoms and NMR parameters are proposed allowingdiscrimination of at least 7 different types of vana-dium sites (tetrahedral sites of Q0, Q1 and Q2 types;trigonal pyramids of 3= 1 and 3= 2 (V2O5-like)types; tetragonal pyramids of 4= 1 and 4= 2 types.

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It is demonstrated that appropriate combination ofdifferent NMR techniques permits now not only toidentify different vanadium sites in crystalline andamorphous materials, but also to get insight into thestructural aspects of disorder and temperature be-havior of different functional groups. The effect ofvanadium atoms in low oxidation states on51V NMRspectra is also discussed.

Acknowledgements

This work was partly supported by RFBR grant01-03-32364 and PAI 04522WK. C. Huguenard andC. Morais are thanked for their help with the STMASexperiment. L.G. Pinaeva, V.N. Bondareva, K.V. Ro-manenko, G.A. Zenkovets, H. Knözinger, M.G. Zuevare thanked for their help and important discussions.

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